[Sync EN] Mark do-not-translate files (#5657)

Commit: `https://github.com/php/doc-en/commit/b513cbbb4245f3c714d046b8556bd43f77443fc3`
PR: `https://github.com/php/doc-en/pull/5657`

**Neue EN-Dateien (noch nicht übersetzt)**
- `appendices/extensions.xml`
- `appendices/license.xml`
- `appendices/reserved.constants.xml`
- `contributors.xml`
- `reference/datetime/timezones.xml`
